Membranes which are selectively permeable to solvent (for example, water) and impermeable to dissolved ions (for example, Na+, Cl−) have been used to desalinate aqueous feed solutions. In one such desalination process—generally referred to as forward osmosis—water is transported from a feed solution through the semi-permeable membrane using a solution on the permeate side of the membrane that has an osmotic pressure that is higher than the osmotic pressure of the feed solution. The driving force for separation in a forward osmosis process is the osmotic pressure gradient through the semi-permeable membrane; because the draw solution on one side of the membrane has a higher osmotic pressure than the feed solution on the other side of the membrane, water is drawn through the semi-permeable membrane from the feed solution to the draw solution to equalize the osmotic pressures of the feed and draw solutions.
Another type of membrane-based desalination is reverse osmosis desalination. In contrast to forward osmosis, reverse osmosis processes use an applied hydraulic pressure as the driving force for separation. The applied hydraulic pressure serves to counteract the osmotic pressure gradient that would otherwise favor water flux from low osmotic pressure to high osmotic pressure.
Membrane-based desalination systems have, to date, been limited by, for example, low efficiencies and have generally been limited to treating waters of relatively low salinities. Improved systems and methods for performing membrane-based desalination are desirable.